Iran has ceased enriching uranium above five percent concentration, International Atomic Energy Agency inspectors confirmed on Monday, as an historic interim deal between the Islamic Republic and world powers over its controversial nuclear program officially began.

The assumption of the Joint Plan of Action is an important milestone for modern Iran, as well as for those concerned about its nuclear ambitions. Developments on Monday marked the first time in over a decade that Iran’s leaders willingly paused, and even began rolling back, aspects of its nuclear work.

Additionally, Iran disabled its centrifuge cascades built to enrich uranium to higher grades, and has begun diluting its stockpile of uranium already enriched to near- 20% concentration.

In a prepared statement, White House spokesman Jay Carney said that “taken together, these concrete actions represent an important step forward.”

Prime Minister Binyamin Netanyahu warned, however, that the Joint Plan of Action would not prevent Iran from implementing its intentions to create nuclear weapons. In a speech welcoming visiting Canadian Prime Minister Stephen Harper to the Knesset, Netanyahu said that stopping the Iranians from gaining the capacity to build a nuclear weapon must still be the international community’s goal.

Enriching uranium to 20% – the point at which uranium is considered “highly enriched,” and the minimum requirement for implosion design – is considered the hardest part of the enrichment process, unnecessary for purposes of civilian power but required for the construction of a warhead. The IAEA is tasked with ensuring not only that Iran disables its centrifuge cascades producing near-20%-enriched material, but also that it begins the dilution process.

Iran is required to convert half of its stockpile into gas form within three months, and to fully dilute that amount by the end of the deal’s six-month time frame.

Netanyahu likened the manufacturing of the fissile material needed to make a bomb to a train that must pass through three stops: the first stop of enriching uranium to 3.5%, the second stop of enriching uranium to 20%, and the final step of enriching uranium to 90% concentration.

“The agreement in Geneva did away with the 20% stop, but left the train on its track and enables Iran to upgrade the locomotive by developing new centrifuges, so that when the day comes, it can leap in a very short time to the final stop on an express track without stopping at an intermediary stop,” he said.

The final agreement that the world powers negotiate with Iran must take the “Iranian nuclear train off the tracks,” Netanyahu said, adding that Iran must not be allowed to have the capability to manufacture a bomb.

The prime minister also said that the international community, at a time when it is relieving sanctions and giving Iran legitimization, should be demanding that Tehran end its calls for the destruction of Israel, and its arming of terrorist groups such as Hamas, Hezbollah and Islamic Jihad.

Harper also addressed Iran, saying that Canada has long held the view that every diplomatic step needed to be taken to keep the Iranians from a bomb, and that as a result, Ottawa appreciated the efforts of the world powers to find a diplomatic solution.

At the same time, he added, “Canada will evaluate the success of this approach not on the merits of its words, but on the implementation and verification of its promised actions.”

Harper said he hoped it would be possible to “walk the Iranian government back from taking the irreversible step of manufacturing nuclear weapons. But for now, Canada’s own sanctions will remain fully in place. And should our hopes not be realized, should the present agreement prove ephemeral, Canada will be a strong voice for renewed sanctions.”

A senior US administration official, in a phone call with journalists on Monday morning, said that the implementation of the interim deal “increases our confidence that Iran cannot break out to a nuclear weapon” without the knowledge of the international community.

A number of senior US officials have briefed their Israeli counterparts by secure video links on the implementation of the deal in recent days, and US sanctions and nuclear experts will travel to Israel this week to continue the briefings in person.

The agreement, reached in November between the Islamic Republic and the P5+1 – the US, United Kingdom, France, Russia, China and Germany – temporarily freezes Iran’s enrichment of uranium above 5% concentration, and its production of a heavy-water plutonium plant in Arak, for $6-7 billion in sanctions relief doled out in increments over the next six months.

The first $550 million installment of restricted funds will not be available to Iran until February 1. But the US began easing some sanctions on the Islamic Republic on Monday, including some against Iran’s oil sector.

The US has vowed to pause its campaign to further reduce Iran’s oil exports from the six countries still importing Iranian crude, administration officials said, beyond the existing sanctions regime that has brought its exports down by 60% in two years.

Wendy Sherman, under secretary of state for political affairs and the chief US negotiator with Iran in the P5+1 talks, traveled to Geneva on Monday to discuss with her international counterparts how best to proceed with negotiations toward a comprehensive solution.

“The coming negotiation to reach a comprehensive agreement that addresses all of the international community’s concerns will be even more complex, and we go into it clear-eyed about the difficulties ahead,” State Department spokeswoman Jen Psaki said in a statement. “But today’s events have made clear that we have an unprecedented opportunity to see if we can resolve this most pressing national security concern peacefully.”

Another senior official issued an explicit warning to companies seeking to renew business in Iran, in violation of existing sanctions still upheld: “Don’t test us.”

The sanctions relief offered in this round of talks will not “come close” to fixing Iran’s economic woes, the official asserted, as existing penalties will continue to bite.